docs: def del_graph(self, name):
          """Remove all traces of a graph's existence from the database
  
          :arg name: name of an existing graph
  
          """
          # make sure the graph exists before deleting anything
          self.get_graph(name)
          self.query.del_graph(name)
          if name in self._graph_objs:
              del self._graph_objs[name]